MAINTAINERS: update Amit Kucheria's email to a single email address

Emails currently go to different mailboxes. Switch to the kernel.org
address so I can forward them to a single mailbox.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Link: https://lore.kernel.org/r/8cbb7004a6a9b846a8d827f514f33f1a265dd5d4.1593498024.git.amit.kucheria@linaro.org


Signed-off-by: Rob Herring <robh@kernel.org>